الفهرس 
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Abstract
Ultrasound signal received from a moving reflector or scatterer has a different frequency than the transmitted ultrasonic signal.this change in frequency is known as the doppler effect.and the magnitude of the change is the doppler shift frequency which is proportional to the velocity of the reflector or the scatterer.for many physiological movements the doppler shifts is in the audible frequency range,a fact which led to the design and manufacture of simple motion detectors in which led to the design and manufacture of simple motion detectors in which the doppler signals are simply fed to headphones or loudspeakers.They have been used for the detection of fetal heart beats and vessel wall vibration in blood pressure monitors.The main area of interest however is the detection and measurement of blood flow in this case the ultrasound is scattered from blood cells.and more sensitive instruments are required to deal with the small received ultrasound signals.The simplest ultrasonic doppler device which is known as a continous wave instrument is sensitive to blood flow anywhere in the ultrasound beam but it cannot easily be used to discriminate between vessels at different depths or to measure the different blood velocities in one vessel.In order to achieve this a repetitive pulse amplitude modulation is usually used,and a device using this is known as pulsed doppler instrument.